Line a 9- by 13-in. pan with plastic wrap, leaving an overhang on the 9-in. sides, and coat with cooking-oil spray. In a large bowl, stir together oats, oat bran, flax seeds, flour, baking powder, salt, and dates until well blended.
In the bowl of a stand mixer, beat together tahini, milk, egg, honey, lemon zest, and lemon juice until well blended.
Add flour mixture to tahini mixture and beat until completely blended. Scrape dough into pan and, with wet fingers or a rubber spatula, pat to fill pan completely and evenly (dough is sticky, so you may need to wash your hands a few times). Chill dough until firm, about 30 minutes.
Meanwhile, preheat oven to 30
Invert pan onto a work surface, lift off pan, and peel off plastic. Using a bench scraper or knife, cut straight down lengthwise through middle, then crosswise to make 16 bars, each 1 1/2 in. wide.
Place bars about 1 in. apart on a baking sheet lined with parchment.
Bake bars until lightly browned and somewhat firm to touch, about 20 minutes.
Remove from oven and let cool completely.
Make ahead: Store airtight up to 2 weeks or freeze.
